Switching power converters with synchronous rectification utilize switching between inductor's current continuous and discontinuous conduction operation modes (CCM and DCM) in order to achieve improved power efficiency across wide load and input voltage ranges. The inductor current zero crossing is sensed in order to detect the operation modes transition point between CCM and DCM. The challenges associated with this include inductor current zero crossing point sensing accuracy, noise effect near the zero crossing point and the sensing circuitry speed and power loss. Moreover, additional hardware such as ADC is required if the used controller is a fully digital controller.
While synchronous rectification is used in switching power converters in order to improve the power efficiency at high load currents in addition to reducing the output voltage ripple, transitioning from CCM to DCM at lighter load currents is used in order to improve light load power efficiency (as explained in references [1-4] listed on page 12). DCM emulation, as known in the art, is a technique used to improve power converter efficiency at lighter load currents while achieving higher efficiency at higher load currents by employing Synchronous Rectifiers (SRs) that operate at CCM. Usually, the DCM operation at lighter load currents is combined with variable or/and reduced switching frequency operation for larger efficiency improvement. Light load efficiency is desirable because new and future power managed load technologies operates most of the time (up to 90% of the time) at light load requirements. Achieving high efficiency across wide load ranges is desirable because the new and future generation of devices is highly power managed and they operate across wide load range and operate most of the time at lighter load currents. Therefore, SR CCM and DCM operations are important for lower power consumption.
Switching from CCM to DCM or vice versa requires sensing the zero crossing of the inductor current. This requires sensing the instantaneous inductor current information at the switching frequency speed, its accuracy and stability is sensitive to noise effect at the zero crossing transition point and it results in additional power losses by the sensing circuitries especially at higher load currents and at higher switching frequencies. The detection of the inductor current zero crossing, which determines the operation mode (CCM or DCM) and determines the SR switch turn off time in DCM, requires sensing the instantaneous current of the output inductor, where the high bandwidth sensing and detection circuitries are essential in order to detect the correct zero crossing point. Moreover, the switching noise, which is introduced at the instances of turning ON and OFF of the converter switches (where the zero crossing of inductor current occurs), makes it more difficult to detect the zero crossing time (and the non zero crossing) accurately. Furthermore, the sensing and detection circuitries power losses and sensing accuracy will impact the controller operation accuracy and impact the power conversion efficiency.
A converter with, for example, a 100 kHz+ or 100 MHz+ switching frequency requires very high-bandwidth low-noise sensing and comparator circuitry to detect the correct inductor current zero crossing for correct DCM and CCM operations. This implies high power loss through the sensing (which may include a sensor and an amplifier) and comparator circuitries. This power loss scales with the switching frequency, and therefore, the higher the switching frequency is (in the future), the higher the sensing power loss. This also impacts the cost and size of the controller. Moreover, the operation may be more impacted by the sensing accuracy at such frequencies as mentioned earlier. This is true in the case of using analog controller, digital controller, or mixed analog-digital controller. Moreover, if the implantation utilizes a fully digital controller, an additional ADC (Analog-to-Digital Converter) may be required which means additional size, cost, and power consumption (as explained in references [6, 7] listed on page 12).
Variable switching frequency is usually used during the DCM operation in order to gain further efficiency improvement (as explained in references [1-4] listed on page 12). Conventionally, the switching frequency is varied as a function of the load current or as a function of a variable that indicates the load current change such as the upper switch (control switch) duty cycle in a buck converter (or the voltage—mode closed-loop compensator error signal) (as explained in references [1-4] listed on page 12). Especially during the CCM operation, when the SR switch gate control signal is complementary to the control switch gate control signal, sufficient fixed or variable SR dead-time should be provided. Several SR dead-time control schemes are known in the art (as explained in references [5-7] listed on page 12). Adaptive SR dead-time control schemes are such those adjusting SR dead-time based on the input current minimization (as explained in reference [7] listed on page 12) or based on the duty cycle minimization (as explained in references [6] listed on page 12).
A need therefore exists for power converter control systems and methods that do not require sensing the inductor current or any current in the converter which eliminates or reduces challenges associated with inductor current sensing for the zero point detection.